remove superfluous includes

- reduce compile time by a little bit (full-core: ~0.7s for me)
- thanks to 'include-what-you-use' for some help
- include Standard C header files before any other (coding style)

src/variable.c

@@ -4,14 +4,12 @@
 ** See Copyright Notice in mruby.h
 */
 
+#include <ctype.h>
 #include "mruby.h"
 #include "mruby/array.h"
 #include "mruby/class.h"
 #include "mruby/proc.h"
 #include "mruby/string.h"
-#include "mruby/variable.h"
-#include "error.h"
-#include <ctype.h>
 
 typedef int (iv_foreach_func)(mrb_state*,mrb_sym,mrb_value,void*);